This appendix documents the time format specifications understood by the :format keyword in time output format specifiers (see Formats). Essentially, it is a reproduction of the man page for GNU strftime function.

Ordinary characters placed in the format string are reproduced without conversion.  Conversion specifiers are introduced by a % character, and are replaced as follows:

%a   The abbreviated weekday name according to the current locale.

%A   The full weekday name according to the current locale.

%b   The abbreviated month name according to the current locale.

%B   The full month name according to the current locale.

%c   The preferred date and time representation for the current locale.

%C   The century number (year/100) as a 2-digit integer.

%d   The day of the month as a decimal number (range 01 to 31).

%D   Equivalent to %m/%d/%y.

%e   Like %d, the day of the month as a decimal
number, but a leading zero is replaced by a space.

%E   Modifier: use alternative format, see below
(see conversion specs).

%F   Equivalent to %Y-%m-%d (the ISO 8601 date format).

%G
The ISO 8601 year with century as a decimal number.
The 4-digit year corresponding to the ISO week number (see %V).  This
has the same format and value as %y, except that if the ISO week
number belongs to the previous or next year, that year is used instead.

%g   Like %G, but without century, i.e., with a 2-digit
year (00-99).

%h   Equivalent to %b.

%H   The hour as a decimal number using a 24-hour clock
(range 00 to 23).

%I   The hour as a decimal number using a 12-hour clock
(range 01 to 12).

%j   The day of the year as a decimal number (range 001
to 366).

%k   The hour (24-hour clock) as a decimal number (range
0 to 23); single digits are preceded by a blank.
(See also %H.)

%l   The hour (12-hour clock) as a decimal number (range
1 to 12); single digits are preceded by a blank. (See also %I.)

%m   The month as a decimal number (range 01 to 12).

%M   The minute as a decimal number (range 00 to 59).

%n   A newline character.

%O   Modifier: use alternative format, see below
(see conversion specs).

%p   Either AM or PM according to the given time
value, or the corresponding strings for the current locale.  Noon is
treated as pm and midnight as am.

%P   Like %p but in lowercase: am or pm
or a corresponding string for the current locale.

%r   The time in a.m. or p.m. notation.  In the POSIX
locale this is equivalent to %I:%M:%S %p.

%R   The time in 24-hour notation (%H:%M).  For a
version including the seconds, see %T below.

%s   The number of seconds since the Epoch, i.e., since 1970-01-01 00:00:00 UTC.

%S   The second as a decimal number (range 00 to 61).

%t   A tab character.

%T   The time in 24-hour notation (%H:%M:%S).

%u   The day of the week as a decimal, range 1 to 7,
Monday being 1.  See also %w.

%U   The week number of the current year as a decimal number,
range 00 to 53, starting with the first Sunday as the first day of
week 01.  See also %V and %W.

%V   The ISO 8601:1988 week number of the current year
as a decimal number, range 01 to 53, where week 1 is the first week
that has at least 4 days in the current year, and with Monday as the
first day of the week.  See also %U and %W.

%w   The day of the week as a decimal, range 0 to 6, Sunday
being 0.  See also %u.

%W   The week number of the current year as a decimal number,
range 00 to 53, starting with the first Monday as the first day of week 01.

%x   The preferred date representation for the current locale
without the time.

%X   The preferred time representation for the current locale
without the date.

%y   The year as a decimal number without a century (range 00 to 99).

%Y   The year as a decimal number including the century.

%z   The time-zone as hour offset from GMT.  Required to
emit RFC822-conformant dates (using %a, %d %b %Y %H:%M:%S %z)

%Z   The time zone or name or abbreviation.

%+   The date and time in date(1) format.

%%   A literal % character.

Some conversion specifiers can be modified by preceding them by the E or O modifier to indicate that an alternative format should be used.  If the alternative format or specification does not exist for the current locale, the behaviour will be as if the unmodified conversion specification were used.  The Single Unix Specification mentions %Ec, %EC, %Ex, %EX, %Ry, %EY, %Od, %Oe, %OH, %OI, %Om, %OM, %OS, %Ou, %OU, %OV, %Ow, %OW, %Oy, where the effect of the O modifier is to use alternative numeric symbols (say, roman numerals), and that of the E modifier is to use a locale-dependent alternative representation.